Mix all the ingredients for the spice mix together. For the dry seasoning, mix 2 tbsp sea salt with the sugar and half the spice mix. Sit the turkey in a roasting tin and gently push your fingers under the skin, starting from the neck, until you can push your whole hand down the length of the breast. Season the bird all over with the dry seasoning, inside and out, and under the skin. Leave in the tin, breast-side up, and put in the fridge for at least 1 hr, or up to two days. If it doesn’t come into contact with anything else, leave it uncovered – this will dry out the skin and make it crispier.
Put the remaining spice mix and the baste ingredients in a saucepan and heat until the sugar has melted, then set aside. Remove the turkey from the fridge an hour before cooking. Heat the oven to 180C/160C fan/gas 4. Work out the cooking time based on 40 mins per kg. A turkey this size should take 3½ hrs, plus 30-45 mins resting. Smear the turkey all over with the butter and put the herbs in the cavity. Loosely cover with foil and roast for the calculated time. After 30 mins, pour over the spiced baste, then baste again every 30 mins. With 1 hr remaining, remove the foil. Put the carrots in the tin, stir to coat in the juices and continue to cook.
Check the turkey with a cooking thermometer – it should read 65C. Leave to rest on a warm platter – it will stay warm for about 1 hr. Continue to cook the carrots until tender. Serve the turkey garnished with fresh bay and thyme, and the carrots alongside.